It would be expensive -- you'd have to "rebalance" the data by copying it to the new volume / away from the old volume, which would take hours or days.
People who want this feature are mostly frugal. They want it so they can upgrade by just buying a single new disk.
That is a rare enough use case that the performance penalty would be worth it.
Frugal enough to shut off their computer when they're not using it (to "save power"), so the rebalancing won't have time to complete in the background?